Heat oven to 425°F. With a vegetable peeler, remove 6 strips of lemon zest, avoiding the white pith; thinly slice crosswise. In a small bowl, combine butter, garlic, mustard, lemon zest, and 1/2 teaspoon each of salt and pepper; set aside.
Remove skin from turkey-breast halves and set aside. Rub the butter mixture all over the turkey breasts. Place one breast half on top of the other, arranging so 1 thick side and 1 thin side are sandwiched together. Wrap breasts in the skin and tie them with butcher's twine to form a log shape.
On a large rimmed baking sheet, toss shallots with oil and 1/4 teaspoon each salt and pepper. Arrange them in the center of the sheet and place the turkey on top. Roast for 60 minutes.
Remove from oven and transfer turkey to cutting board. Add potatoes, carrots, and parsnips to a baking sheet and toss with shallots and 1/2 teaspoon each of salt and pepper; arrange in an even layer. Place turkey on top of vegetables and roast until turkey registers 160°F on an instant-read thermometer and vegetables are golden brown and tender, 35 to 45 minutes more (remove vegetables that are done before turkey).
Transfer turkey to a clean cutting board and let rest for at least 10 minutes before slicing. Serve with pan juices and vegetables.